The aforesaid U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position of a camshaft relative to a crankshaft. The aforesaid co-pending application Ser. No. 713,465 further describes a VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing, the vane being oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system for the VCT system of U.S. Pat. No. 5,002,023 utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure on an end of the spool and the relationship between the hydraulic force on such end and an oppositely directed mechanical force on the other end which results from a compression spring that acts thereon.
A problem with the control system of the aforesaid type is that it relies on an hydraulic force of variable magnitude to counteract a mechanical force. This problem arises from the fact that the pressure and viscosity of the hydraulic fluid that is used in the control system, illustratively engine lubricating oil in an automotive VCT application, can change over a period of time due to changes in the engine r.p.m., the operating temperature or age of the oil, or variations in the composition of the engine oil from time to time as a result of an oil change in which the old oil is replaced by an oil of a different brand or grade. In any case, in a control system Of the aforesaid type the actual hydraulic control pressure, which is at least partly related to viscosity in a dynamic system, is maintained at a predetermined value by changing the duty cycle of a pulse width modulated (PWM) solenoid. The PWM solenoid, which is included in a control system of the aforesaid type, is used to control the hydraulic pressure at a reduced level from a higher pressure source, for example, the engine oil gallery, based on the duration of the "on" cycles of the PWM solenoid relative to its "off" cycles. Very sophisticated software is required to control the duty cycle of a PWM solenoid to prevent changes in engine oil pressure or viscosity from undesirably changing the desired centered or null position of the control valve spool.